net-im/{mtxclient/nheko}: Update to latest upstream

New mtxclient and related nheko releases are at
https://github.com/Nheko-Reborn/mtxclient/releases/tag/v0.8.0
https://github.com/Nheko-Reborn/nheko/releases/tag/v0.10.0

This chases some Matrix protocol updates as well. Drops Matrix
groups support. VOIP might be supported, I haven't tried it.

Nheko doesn't do a build-time check for the required mtxclient
version, so it will try and fail to compile against mtxclient 0.7.
Hence, committing these two together (I wish I could specify
versions better in LIB_DEPENDS).